Advanced Combat Optical Gunsights (ACOG) are a series of telescopic sights designed and used on the M16 rifle and M4 carbine. The ACOG's reticle is illuminated by an internal phosphor illuminated by the radioactive decay of tritium. The sights are standard issue for the United States Army and United States Marine Corps for use in connection with the M16 and M4 and are utilized for target acquisition and shooting accuracy under stressful conditions such as combat. Modern day optical sights are extremely delicate and prone to failure under harsh conditions especially in cases such as when the firearm is dropped, repeated recoil and rough handling in general.
When the principal optical sight fails, there is generally insufficient time to replace the sight and, therefore, the need arises for an auxiliary backup sight which is unobtrusively mounted on the firearm when not in use and easily and quickly deployable as needed when the primary optical sight becomes inoperable.